Caption: Light microscopy of lung tissue showing a bronchus. As large diameter airways, bronchi are prevented from collapse by the reinforcement of their walls with cartilage stained blue. The cartilage forms incomplete rings around the airway. In contrast, the smaller bronchioles do not have cartilage. Large pulmonary blood vessels are prominent. Most of the lung tissue is seen as air-filled alveoli where gas exchange occurs between them and capillaries within their very thin walls. Magnification x15 when printed at 10 cm.